FIG. 3 is a block diagram of a conventional type of cordless telephone set (a first example of conventional technology). This example of conventional technology was disclosed in Japanese Patent Laid-Open Publication No. 326647/1992. In this figure, the reference numeral 301 indicates a parent telephone set, while the reference numeral 302 indicates a slave telephone set. The parent telephone set 301 has basically the same configuration as that of the slave telephone set 302, and brief description is made below for the basic configuration of the parent telephone set 301. The parent telephone set 301 is provided with a speech network IC 314, and connected to the speech network IC 314 is a telephone line LINE, and also connected thereto is a receiver 316 having a parent telephone set microphone as well as a parent telephone set speaker.
Also in the speech network IC 314 is incorporated a sidetone generator to generate a so-called sidetone, and when setting a call using the receiver 316 in the parent telephone set 301, a caller can hear a portion of voice taken from a parent telephone set microphone as sidetone, so that harmonious call is possible.
Also connected to the speech network IC 314 is a secret talk processing circuit 313. Specifically, the secret talk processing circuit 313 is provided with a secret talk processing section and a secret talk reproducing section, both of which include an A/D convertor and a D/A convertor respectively and execute a digital secret talk processing such as, for instance, interleave.
In addition to the basic configuration as described above, in this example of conventional technology, the parent telephone set 301 has a sidetone canceling circuit 315, while the slave telephone set 302 has a sidetone generating circuit 324. This configuration is employed because sidetone in the slave telephone set is generated by a sidetone generator provided in the speech network IC 314 of the parent telephone set 301, and for this reason a large delay time is generated in a secret talk processing circuit existing in a path from the slave telephone set 302 to the parent telephone set 301 as well as in a path from the parent telephone set 301 to the slave telephone set 302. Hence, a person using the slave telephone set 302 monitors the person's own voice delayed by the delay time described above, causing a serious disturbance because of echo.
Namely the sidetone generating circuit 324 in the slave telephone set 302 outputs a portion of aural signal, obtained through a microphone of the slave telephone set as a sidetone, to a speaker of the slave telephone set, while a sidetone canceling circuit 315 in the parent telephone set 301 deletes the slave telephone set sidetone generated in the sidetone generating circuit 324. With this feature, delay because of digital secret talk is eliminated, and a talker using the slave telephone set 302 can monitor the talker's own voice without experiencing echo disturbance.
A telephone set employing therein a speech network IC in a speaker circuit is called "a telephone set based on a speech network IC system". In contrast to it, a telephone set using a transformer circuit such as a hybrid transformer in the speaker circuit is called "a telephone set based on a transformer system".
FIG. 4 is a block diagram of a parent telephone set in a conventional type of digital cordless telephone using the transformer system (second example of conventional technology). The parent telephone set in this example of conventional technology is provided with a phase shifter 404 and a mixer 402, each as a sidetone level reducing means, in addition to a secret talk processing circuit 123 and a hybrid transformer 121. This sidetone level reducing means combines, by the mixer 402, an aural signal for calling subjected to phase inversion in the phase shifter 404 with that returning as a sidetone from the telephone line section to cancel the sidetone.
Next, description is made for problems in the transformer system (second example of conventional technology), taking into consideration the first and second examples of conventional technology described above. At first, FIG. 5A is a block diagram illustrating a case where a telephone set (a parent telephone set) based on the speech network IC system is connected to a telephone line. A speech network 502 in this figure is, for instance, TEA 1067, and has a resistance bridge type of sidetone canceling circuit, an AGC (Automatic Gain Control) circuit, and an amplifier to compensate degradation of the receiving characteristics.
In this case, the AGC circuit in the speech network IC 502 detects a current i flowing in the line and controls a gain in amplifiers 505, 506 of the telephone set. The line loss varies according to type of line and line length, but with this configuration, a telephone set which can respond to various types of line can be realized.
FIG. 5B is a block diagram illustrating a case where a telephone set (a parent telephone set) based on the transformer system is connected to a telephone line. Unlike the AGC circuit in the speech network IC 502, the hybrid transformer 512 does not have a means for controlling gains in the amplifiers 515 and 516 in the telephone set by detecting the current i flowing in the line. For instance, assuming that a standard sound is 94 dBSPL and a line loss is 10 dBSPL, a gain of 10 dB is required for the microphone amplifier 515 and the speaker amplifier 516 respectively.
Line loss varies according to line type and line length, but, for instance, in a case where a telephone set having the configuration as shown in FIG. 5B is applied to a local exchange having a low line loss or the like, a transmitting level and a receiving level in a telephone set (a parent telephone set) become high, and naturally the level of sidetone returning through the parent telephone set to be heard by a talker using the slave telephone set also becomes higher, which causes some problems in hearing through the slave telephone set. Also, even if the configuration in the first example of conventional technology (as disclosed in Japanese Patent Laid Open Publication No. 326647/1992) is applied, gains of the amplifiers 515 and 516 in the telephone set remain relatively large, so that it is impossible to lower the level of sidetone.
As described above, in the conventional type of radio transmitter/receiver, especially based on the transformer system, gain control for an amplifier according to a current flowing in a line is not executed, so that level of sidetone can not be reduced, and also a telephone set which can respond to various line types and different line lengths can not be realized.